The spine carries very high vertical loads within the body with a structure made up of a series of individual vertebrae spaced apart by intervertebral disks therebetween. The vertebrae are rigid in form while the disks exhibit a certain degree of flexibility and elasticity.
A common spinal failure involves failure or fracture of the pars interarticularis. The resulting spinal displacement, called spondylolisthesis, typically causes extreme pain and in some cases other neurologic and health problems.
A common treatment for such spondylolisthesis is for the disk to be removed and for the two vertebrae on either side of the intervertebral disk space to be fused together. Such a spinal fusion procedure first involves removal of the disk from the intervertebral disk space. Next, some form of structure is placed between the vertebrae and within the disk space to distract the two vertebrae away from each other; pedicle screws and rods are then inserted to hold the vertebrae in place. Finally, bone in-growth media is placed around this implant, to cause bone to grow between the two vertebrae and through the disk space, causing the two vertebrae to fuse together. The implant holds the vertebrae in place until the new bone has formed to fuse the vertebrae together. Once fusion is complete, new bone material within the disk space takes over the load carrying function within the disk space, with the implant remaining and providing some additional stability to the spine.
Typically, the implant placed within the disk space does not couple the two vertebrae together, but rather only fills the disk space and distracts the two vertebrae away from each other. The vertebrae are held adjacent to each other by pedicle screws and other bodily structures which remain in place to hold the adjacent vertebrae together.
The combination of intervertebral disk space implants, also commonly referred to as “cages” and the use of spinal stabilization rods and pedicle screws is effective in many cases for supporting adjacent vertebrae during spinal fusion. However, in other cases the degree of stabilization is insufficient and the spinal fusion procedure is less than completely successful, leaving the patient with ongoing pain and loss of function.
The degree of success of such prior art spinal fusion procedures varies to some degree depending on where along the spine the fusion is to take place. One portion of the spine where prior art cages, spinal rods and pedicle screws have been particularly ineffective is at the disk space between the sacrum and the lumbar vertebra adjacent the sacrum, often referred to as the L5 vertebra. The way that the sacrum angles away from a centerline of the spine makes it difficult for the implantation of spinal rods and cages, particularly in the setting of spondylolisthesis, and even if implanted, the angle makes it difficult for the forces involved to be effectively supported by such prior art vertebrae stabilization implants. Accordingly a need exists for a spinal stabilization implant which is particularly effective in holding the sacrum and the L5 vertebra fixed together while fusion takes place across the disk space therebetween.
One family of prior art implants that is known for stabilizing the lumbar vertebra and the sacrum are described in U.S. Pat. Nos. 6,558,386; 6,899,716; 6,921,403; 7,087,058; and 7,309,338. These implants require an elaborate caudal approach to the lower spine.
Spondylolisthesis occurs when two adjacent vertebrae become displaced in an anterior or posterior direction relative to each other. Spondylolisthesis can occur to varying degrees with the degree of severity corresponding with the need for repair by fusing the vertebrae together. A need exists for an implant which can secure the L5 vertebrae to the sacrum (or other two adjacent vertebrae) sufficiently that spondylolisthesis can be treated through spinal fusion effectively, simply and with a minimum of implants and with either an anterior or posterior approach.